A conventional example of this kind of techniques is a zigzag-slab solid-state laser apparatus disclosed in Non-patent Document 1. This solid-state laser apparatus circulates a coolant through a laser medium, so as to prevent the laser medium from raising its temperature. This is done in order to prevent the pumping light generated by a semiconductor laser from raising the temperature of the laser medium and causing a thermal lens effect and the like.    Non-patent Document 1: “Amplification Analysis of High-Output LD-Pumped Zigzag-Slab Nd Glass Laser”, Digest of Technical Papers, the 23rd Annual Meeting of the Laser Society of Japan, p. 51